A relief valve of a fuel supplying system disclosed in, for example, the patent literature 1 is known.
Specifically, in the structure disclosed in the patent literature 1, an inlet portion receives the fuel, which is branched from a supply flow of the fuel directed to the internal combustion engine, and an outlet portion outputs the fuel, which is inputted into the inlet portion, to the outside. The inlet portion and the outlet portion are connected with each other through a connecting portion. A valve element (in the patent literature 1, a combination of a valve body and a movable holder), which is received in an inside of the connecting portion, is seated and lifted relative to a valve seat, which is formed in the inlet portion and is located on an upstream side of the valve element, in a state where the valve element is urged toward the valve seat by a resilient member. Here, a valve opening pressure for opening the valve element away from the valve seat is determined according to a resiliency characteristic that is preset at the resilient member. Therefore, at the valve closing time, the pressure of the supply fuel directed toward the internal combustion engine side becomes lower than the valve opening pressure, and thereby the valve element is seated against the valve seat. At this time, the fuel flow from the inlet portion to the outlet portion is blocked, so that the pressure of the fuel can be maintained. In contrast, at the valve opening time, the pressure of the supply fuel directed toward the internal combustion engine becomes equal to or larger than the valve opening pressure, and thereby the valve element is lifted away from the valve seat. At this time, the fuel flow from the inlet portion to the outlet portion is enabled, so that the pressure of the fuel can be released.
In the structure recited in the patent literature 1, a portion of the valve element, which is loosely inserted into the inside of the connecting portion on the downstream side of the valve seat, enables communication of a communication passage, which is located between the portion of the valve element and the connecting portion, to the inside of the inlet portion. The inventors of the present application have extensively studied the fuel flow, which is generated in the inside of the connecting portion at the valve opening time. As a result of the study, the inventors of the present application have found that a swirl flow of a low flow speed is generated on a downstream side of the valve element that is an opposite side of the valve element, which is opposite from the valve seat, relative to a main flow of a high flow speed that is directed from the communication passage toward the downstream side in the axial direction of the valve element. The swirl flow of the low flow speed will cause a change in a back pressure, which is applied from the downstream side to the valve element placed in the valve opening state. Therefore, it will result in chattering of the valve element, and thereby pressure pulsation is generated in the fuel. Furthermore, the inventors of the present application have also found the following phenomenon. That is, at the location between the valve element and the valve seat, at which the negative pressure is exerted in the valve opening state of the valve element, fuel vapor is generated due to the phenomenon of boiling in the vacuumed state. The thus generated vapor is carried by the main flow or the swirl flow. Thereby, the resilient member, which urges the valve element, is vibrated due to the collision of the vapor against the resilient member. The pressure pulsation and the vibration will cause generation of a noise in the path to the internal combustion engine. Therefore, it is desirable to limit the generation of the pressure pulsation and the vibration.